Affiliate networks that already have several advertisers typically also have a large pool of publishers. These publishers could be potentially recruited, and there is also an increased chance that publishers in the network apply to the program on their own, without the need for recruitment efforts by the advertiser.
Relevant websites that attract the same target audiences as the advertiser but without competing with it are potential affiliate partners as well. Vendors or existing customers can also become recruits if doing so makes sense and does not violate any laws or regulations.
Almost any website could be recruited as an affiliate publisher, although high-traffic websites are more likely interested in (for their own sake) low-risk cost per mille or medium-risk cost per click deals rather than higher-risk cost per action or revenue share deals.